An optical sensor has such functions as (i) a function of detecting a detection target and (ii) a function of detecting, for example, the distance between a detection target and itself. Optical sensors are thus finding applications in an increasingly wide field.
Electronic devices such as a mobile phone (including a smartphone) and a digital camera include a liquid crystal panel for displaying an image. Some models of such electronic devices include a touch panel that allows a user to perform a touch operation on the liquid crystal panel. Further, some mobile phones include, in an audio output section to which the user brings an ear, a proximity sensor mounted for detecting an approach of a human face to the mobile phone. Such a proximity sensor is used to, for example, allow the touch panel to be turned off in a case where a human face approaches the liquid crystal panel. The proximity sensor thus serves to reduce power consumption by the mobile phone and to prevent a malfunction of the touch panel.
In a technical field of a sensor for detecting a movement of an object, there is a demand for use of an optical sensor as, other than a proximity sensor as described above, a gesture sensor for detecting a movement of a human hand. Such a gesture sensor detects a movement of a hand over the touch panel in a noncontact manner. This allows the user to operate the liquid crystal panel so as to scroll screen contents thereon without dirtying the surface of the liquid crystal panel with a wet or dirty hand. A gesture sensor further allows the user to operate the liquid crystal panel normally even in a case where the user wears a glove or the like, in which case the touch panel usually cannot detect a touch operation easily.
Patent Literature 1, for example, discloses a reflective optical sensor as an optical sensor for detecting a movement of an object. This optical sensor, as illustrated in FIG. 9, includes a light emitting element 301 and two light receiving elements 302 and 303, with (i) the light receiving element 302 disposed on one side of the light emitting element 301 and (ii) the light receiving element 303 disposed on the other side thereof. In a case where a detection target object 304 is on the right side, light reflected from the detection target object 304 strikes the light receiving elements 302 and 303 in a large amount. On the other hand, in a case where the detection target object 304 is on the left side, light reflected from the detection target object 304 strikes the light receiving elements 302 and 303 in a large amount. The optical sensor calculates a difference between (i) a photocurrent generated in the light receiving elements 302 and (ii) a photocurrent generated in the light receiving element 303, and thus detects the position and movement of the detection target object 304.
Patent Literature 2 discloses a gesture sensor including optical sensors in multiple segments and a control circuit for processing a current outputted from each optical sensor.